THE EFFECT OF THE TIMING OF POSTDIAPAUSE EGG DEVELOPMENT ON SURVIVAL,GROWTH, AND BODY-SIZE IN GRYLLUS-PENNSYLVANICUS

A trade-off between early mortality and reproduction may influence the evolution of the timing of post-diapause development in insects that exploit seasonal environments. To test this prediction, we manipulated hatching date experimentally in three successive years within a popul ation of Gryllus pennsylvanicus, a univoltine cricket that overwinters in the egg stage. Each year, cohorts of nymphs derived from eggs main tained under identical conditions in the laboratory were simultaneousl y raised in the laboratory and in the field from where the population was collected. In the field, nymphal mortality was high early in the s eason, but was independent of hatching date in the laboratory under fa vorable rearing conditions. This suggests that the low temperatures en countered by the nymphs early in the season had a detrimental effect o n survival. Growth rate and adult body size decreased with a delay in hatching date in the laboratory, which suggests that delayed hatching had a direct effect on the timing and rate of subsequent life history processes. The nymphs in the laboratory developed under a long and con stant photophase indicating the onset of summer, whereas nymphs in the field experienced a declining photophase. Such a difference in photop eriod apparently induced a plastic response in growth rate, with nymph s growing faster under the declining than the constant photophase. It also appears that the declining photophase reduced the degree-days req uired for metamorphosis in the cohorts, and induced a decline in adult size with development time within each cohort. In either case, such a n accelerating effect on metamorphosis implies a decrease in fecundity with a delay in hatching date. Therefore, these findings provide evid ence for the hypothesis that a trade-off between early mortality and r eproduction influences the evolution of the timing of post-diapause de velopment at this locality.